An optoelectronic module typically comprises one or more optoelectronic components, such as light emitting diodes (LEDs), lasers, detectors, optical modulators, and one or more electronic chips, such as CMOS, BiCMOS, GaAs, or any other technology to used to fabricate integrated circuits. At least one of these electronic chips is connected electronically to at least one of the optoelectronic components. The connection may be made using wire bonding, flip-chipping, or any other suitable technique for achieving electrical connection.
Typically, in addition to an electrical connection, an optical interface is provided on the optoelectronic module, which provides an interface from the optoelectronic module to an external optical system. The external optical system can be a connector with optical fibers, an optical sensing system, or light emitting system.
The optoelectronic modules need to be packaged. A package for the optoelectronic modules needs to provide an optical interface for connecting to an external optical system, so that an optical signal to be obtained or received can be coupled out. The optical interface fulfills two functions. First, the optical interface facilitates a precision alignment from the optoelectronic module to the external optical system, with accuracy typically in the order of 1 to 10 μm depending on the application. For example, for multi-mode systems the accuracy is typically of the order of 10 μm and for single-mode systems the accuracy is typically in the order of 1 μm. Such accuracy is typically required, for example, in parallel optical interconnect systems.
The precision alignment may be achieved by using external alignment feature structures, such as alignment pins or ferrule accepting reference holes. The use of alignment pins is described in U.S. Patent Application Serial No. 2003/0219217. The use of ferrule accepting reference holes is described in U.S. Patent Application Serial No. 2003/0136968.
A second function of the optical interface is to facilitate guiding of light to and from the external optical system. Optical elements, such as lenses or wave-guides, integrated into the external alignment structure may be used to facilitate the guiding of light.
The optical packaging is employed to assemble the optical elements and the external alignment feature structure on the optoelectronic package with the desired accuracy. For example, micro lens arrays can be positioned in the package, as described in U.S. Pat. No. 6,736,553. As another example, wave-guides can be positioned in the package, as described in U.S. Pat. No. 6,674,948 and U.S. Pat. No. 6,722,788. Typically, the assembly of the package uses alignment features on the optoelectronic module, the optical element, and the external alignment feature structure.
When packaging, it is desirable that the optical element includes alignment features to assure a good positioning of the optical element on the optoelectronic package. This is not a simple task as the fabrication of the optical elements and the fabrication of alignment features on these optical elements typically use different technologies. Whereas the alignment features typically are made using thin-film techniques, the optical components typically are made using other techniques such as etching or molding. The use of these different techniques for fabricating the alignment features and the optical components usually results in some amount of misalignment.
In the assembly phase, the optical element is positioned on the optoelectronic module, again resulting in some misalignment, as well as longer fabrication times and a decreased yield. It is to be noted that in the optoelectronic module, not only the X and Y position, but also the vertical distance, i.e., the Z-alignment, between two objects determines the coupling efficiency. To achieve good Z-alignment, precision mechanical elements, such as spacers, are employed to define the height position of two objects. Using precision mechanical elements is expensive, involves additional handling, and is dependent on the availability of high-precision mechanical parts with the desired accuracy.
In LED packages and similar products, lenses are molded directly in the package, omitting the need for an intermediate optical element. In the same molding step, the alignment features to the external optical system can be defined (for example, to accept the ferrule of the connector). However, such directly molded optical structures on the optoelectronic module, in general, cannot produce accuracy better than 10 nm. Furthermore, this technique is limited to lenses only, and is not suitable for making alignment pins.
The molding process employs high-pressure and temperature to achieve good optical quality of the molded structure, which may not be compatible with the underlying optoelectronic module. Moreover, non-recoverable expenses of the molding process are relatively high, as designing the mold is an iterative and expensive process. Finally, this technique is not flexible, as it does not allow integrating the external alignment features into the package during molding. Furthermore, each design employs a new mold.
Most techniques used for fabricating refractive micro-optical components are based on well-known processes coming from microelectronics, such as standard lithographic techniques combined with a thermal reflow process and, if desired, pattern transfer to a substrate using dry etching or lithography using half-tone (e.g., greyscale), masks, and etching. Other direct-write techniques include laser writing (e.g., with a focused HeCd laser), e-beam or focused ion-beam writing, diamond turning, and micro-jet printing to name a few.
Replication of these structures in large quantities typically is achieved by injection molding, embossing, or casting. Another technique more recently introduced as a feasible fabrication technology, is laser ablation. This technique differs significantly from ‘laser writing’ mentioned above. In the first place, laser ablation does not require resist development as the material is physically removed during ablation. Secondly, while laser writing with a HeCd laser is limited to resists of merely a few microns thick, laser ablation allows machining of surfaces up to several hundreds of microns deep. Finally, it should be noted that pulsed laser ablation, in particular when an excimer or CO2 laser source is employed, involves a beam size that is much larger than the focused submicron spot of a continuous laser writing device, and typically amounts to several tens of microns or larger.
Laser ablation offers some advantages with respect to the previously mentioned fabrication techniques as laser ablation has a direct-write and contactless etching nature. Additionally, laser ablation has the potential to define microstructures and microoptics on a top surface of a heterogeneous optoelectronic module in a very late phase of the assembly process. Several techniques using laser ablation, in particular excimer laser ablation, for microoptics fabrication have been reported. In general, these techniques suffer from a variety of disadvantages, making laser ablation less attractive as prototyping technology.
In Appl. Opt. 36 (1997) p. 4660, Wang et al. suggest using complex mask patterns for fresnel lens fabrication. This method is not cost-effective when lenses with different focal lengths, diameters, or shapes need to be fabricated since every other lens requires different mask patterns. The application of greyscale masks, as suggested by Matz et al. in Appl. Phys. A 65 (1997) p. 349, suffers from the same disadvantage. Mihailov and Lazare in Appl. Opt. 32 (1993) p. 6211 report on the ablation of islands in a polymer substrate, followed by thermal reflow. Although the ablation process can be performed within seconds, reflow of the surface typically takes a few hours, making the technique less suitable for fast prototyping.
Another approach is based on irradiation of the polymer with UV (i.e., wherein no actual ablation is involved) and swelling of the irradiated zone due to diffusion of Styrene in a controlled way. Again, this last process step requires several hours to finish the microlenses. A similar process is based on irradiation of doped PMMA at subablative fluence only, as described by Beinhorn et al. in Appl. Phys. A 68 (1999) p. 709, where spontaneous swelling is the result of a balanced combination of photochemical reactions and surface tension, which means that the process is likely to be very material, dopant, and fluence sensitive.
Other work involves the use of scanning excimer laser ablation for correction of phase aberrations of glass lenses by ablation of a thin resin coating on the lens. However, experimental results report only on maximum ablation depths up to 5 μm. Examples with CO2 laser heating are also reported. None of the above-mentioned techniques combines fast processing with a sufficient machining accuracy and compatibility with heterogeneous assembly processing.
The use of connectors as external optical systems, for example in parallel optical interconnect systems, employs a flat optical interface to allow for a good physical contact between the connector and the optoelectronic module. Standard molding techniques cannot achieve the desired level of flatness, which can be, for instance, 0.1, 0.2, 0.5, 1, or 2 μm. Precision molding of micro lenses is shown in U.S. Pat. No. 6,597,020. These techniques allow for the production of optical elements and are suitable for producing the optical elements that could be assembled onto the optoelectronic module in a later step.
In U.S. Pat. No. 6,722,788, the problem of coupling light from/to an optoelectronic device in a package is described. The capability of out/in coupling of light is provided for using a complex system of connectors and couplers, which carry optical fibers and couple the fiber through the package towards the optoelectronic component, so that the fibers are well aligned with respect to the underlying optoelectronic component. A series of fibers is bundled in a fiber bundle array connector. A coupler holder piece, into which a light collimating or imaging coupler can fit, is fixed to a module connector. The module connector is the latching piece that usually sits permanently on the module, and onto which a fiber bundle array connector can fit. A fiber bundle array connector is then inserted into the module connector piece/coupler holder combination.
Light is shone through the far end of the fiber bundle so that light emits from the fiber array connector end. This light can be used for alignment purposes. The collimated coupler is inserted into the coupler holder and aligned so that the critical optical elements of the collimated coupler are aligned to the individual fibers in the fiber bundle/bundle array connector. Once the two pieces are aligned, the collimated coupler is permanently fixed in place in the coupler holder. This assures that any time the fiber bundle array is removed from and reconnected to the module connector that the individual fibers will still be aligned relative to the collimated coupler.
The assembly containing the collimated coupler, the coupler holder, and the module connector is then aligned relative to the chip assembly so that all of the optical elements in the collimated coupler are aligned relative to all of the optical devices in the chip assembly. This assures that efficient transfer of light between the optical devices and the collimated coupler occurs. The assembly containing the collimated coupler, the coupler holder, and the module connector is then brought into close proximity to the chip assembly and permanently affixed in place. This seals the optical devices, ensures that the alignment between the optical devices and the collimated coupler is maintained, and ensures that the spacing between the optical devices and the collimated coupler is small enough so that cross talk does not take place. This technique is very complex, expensive, and has reliability problems.
Therefore, there is a need for a technique of packaging optoelectronic modules using simple manufacturing techniques, and that results in better accuracy and process compatibility with the underlying optoelectronic module.